Chlorine dioxide is commercially generated by the reduction of an alkali metal chlorate, such as sodium chlorate, with an appropriate reducing agent in a strongly acidic aqueous medium. Common reducing agents include chloride ion, sulphur dioxide and methanol. The strongly acidic aqueous medium is usually provided by a concentrated sulphuric acid, at an appropriate acid normality in the range of from about 3 to about 10 depending on the nature of the generation process employed.
In the method of chlorine dioxide generation using chloride ion as the reducing agent, the chloride ion is most commonly provided by sodium chloride or by hydrochloric acid. However, contributing to the relative chemical inefficiency of this method is the accompanying generation of unwanted chlorine. Also, the high cost of chemicals, i.e. hydrochloric acid, and hazards from "puffing" (explosions caused by decomposition of the chlorine dioxide) detract from the desirability of this method.
The generation of unwanted chlorine can be avoided by employing sulphur dioxide or methanol as the reducing agent in sulphuric acid medium to generate chlorine dioxide. However, the chemical inefficiency of the method in the past in regards the yield of chlorine dioxide generated with respect to chlorate conversion and reducing agent effectiveness, together with the hazards from "puffing" have remained as serious drawbacks.
Canadian Patent No. 434,213, issued April 1946 to The Mathieson Alkali Works, discloses a process for producing chlorine dioxide from chlorate in the presence of sulphuric acid using a restricted (or controlled) amount of an organic, water-soluble reducing agent, such as a water-soluble aldehyde, alcohol, carbohydrate, organic acid, or starch. The invention described therein is predicated upon the amount of reducing agent added being so restricted that over-reduction of the chlorate is substantially avoided, and the danger of explosive reaction is practically eliminated.
U.S. Pat. No. 2,736,636 (1956) of Day et al. discloses the production of chlorine dioxide by the use of increased amounts of reductant with simultaneous utilization of much less quantities of sulphuric acid. Suitable reductants disclosed include organic materials described as alcohols, aldehydes, organic acids and sugars, specifically, wood pulp, kraft pulping process waste liquor, molasses and starch. It is recognized that these materials are highly oxygenated (contain a large proportion of oxygen). Furthermore, U.S. Pat. No. 2,736,636 discloses the separate addition of the reductant to the process described therein.
It has not been described how to improve the efficiency of the method without also increasing the safety hazards incident to "puffing" and high heat generation which would accompany any such increase in efficiency. Nor has it been described how to improve the efficiency of the reaction compatibly with generation of the chlorine dioxide in a form free from the spent or residual materials contained in the liquor.
It is common practice in petroleum refineries to treat low boiling isoparaffins or alkanes with alkylating reagents such as olefins, in the presence of sulphuric acid as a catalyst, to produce a high octane alkylate boiling in the gasoline range. It is well known that the sulphuric acid catalyst, commonly termed alkylation acid, does not undergo major chemical change during the alkylation process but the acid concentration diminishes due to the build-up of water and organic impurities or "red oils" originating from undesired side reactions. As the build-up of the water and red oil diluents approaches 10-12%, the concentration of the alkylation acid is reduced from its original value of about 98.0-99.5% to about 86-90%. At these lower concentration values, the catalytic activity of the alkylation acid is decreased and the octane number of the product alkylate is undersirably lower. At this point the alkylation acid is withdrawn from the system, is known as alkylation waste acid, and is reprocessed by regeneration. This alkylation waste acid typically contains about 3-6% by weight of water and about 3-8% by weight of organic materials. The nature of the organic materials, or "red oils", in the waste acid is complex since they are derived from cyclic and acyclic aliphatic compounds, and includes mixtures of alkanes, alkenes and sulphonated derivatives thereof. In reprocessing, the alkylation waste acid is usually thermally decomposed to sulphur dioxide, carbon dioxide and water. After purification, the sulphur dioxide generated is reconverted to sulphuric acid by the conventional contact process.
Besides the energy required for the acid regeneration, another obvious disadvantage of this widely used destructive regeneration process is that the alkylation waste acid is completely decomposed to eliminate the organic material impurities. Analyses of alkylation waste acid vary somewhat depending on the operating conditions.